1. Field of the Invention
This invention relates to a light transducer and, more particularly, to an automatic servo-positioning system for positioning a scanning beam for use in a videodisc player.
2. Description of the Prior Art
In the sensing of information recorded on a disc, such as a videodisc for use in a videodisc player, positioning of the read beam in relation to a preselected spot on the track being read is quite critical. Deviation from the center may cause the output of the returned beam to be distorted or at too low a level to be sensed.
A videodisc will have information prerecorded in concentric circular or spiral tracks with a 2.mu. radial distance between adjacent tracks with a track width of 1.mu.. In order to follow this track with a disc rotating at 1800 rpm, it is an absolute necessity that the read beam be always accurately positioned over the data track being read.
The prior art has attempted to cope with this problem in a variety of ways. Open loop methods have been used wherein the positioning of a read head was mechanically set and subsequently readjusted. Closed loop systems have also been employed where separate positioning tracks were recorded coincidentally with the data tracks. Specially grooved record discs have also been used wherein the read-recording head was positioned by virtue of its spatial relation to the groove.
In U.S. Pat. No. 3,126,535, to Streeter, a system for tracking magnetic discs was disclosed in which a small transverse oscillatory motion was applied to the magnetic transducer after it has been positioned in the general area of a data track. The oscillating motion of the transducer causes an amplitude modulation of the magnetically recorded signals being read.
The phase of the output signal relative to the phase of the oscillatory motion is dependent upon the relative positioning of the transducer and the recorded track. This provides the necessary indication as to whether the transducer is positioned at its optimum sensing point.
The magnetic head, therefore, is responsive to the aforementioned phase relationship, for changing its position in a direction such as to center it over the track.
As is well known, magnetic discs are generally provided with circular information tracks of a width, orders of magnitude greater than the one micron track employed on the video disc. Further, the mass and inertia of the magnetic head system precludes the use of the approximately 1 kHz oscillating frequency that would be desirable in tracking a videodisc channel.
Further, and in contrast to magnetic recording systems, the videodisc system provides a signal whose average value is at a minimum when the reading beam tracks the center of the information channel. This requires a mechanization that is fundamentally different from that taught by Streeter, supra.